Tab Atkins Jr.:
> CSS3 Values doesn't care what's inside of it either. It's just a
> string. V&U explicitly defines to use 3986 right now to absolutize a
> URL, so specs don't need to care about that detail.
Oh, so because the definitions of properties that use <url> talk about
their behaviour in terms of the value at some point later than the
"computed value" stage, it is already required that it has been
interpreted as a URI per RFC 3986? (Because absolutisation of <url>s
always happens as part of determining the computed value.)